1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the optical scanning apparatus, which are suitable for a laser beam printer, a digital copying machine, a multifunction printer, and the like using an electrophotographic process.
2. Description of the Related Art
Conventionally, an optical scanning apparatus used for an image forming apparatus such as a laser beam printer and a digital copying machine guides a beam emitted from a light source to a deflector by an incident optical system. Then, the beam deflected for scanning by the deflector is imaged by an imaging optical system on a photosensitive drum surface as a surface to be scanned into a spot shape, so that the beam optically scans the photosensitive drum surface.
As the optical scanning apparatus, there is known an optical scanning apparatus that further includes a multibeam light source having a plurality of light emitting points arranged at regular intervals at least in a sub-scanning direction so as to achieve both higher density of images and higher speed (see Japanese Patent Application Laid-Open No. 2008-268721). The apparatus disclosed in Japanese Patent Application Laid-Open No. 2008-268721 uses a scanning lens 11a on a polygon mirror side and a scanning lens 11b on the photosensitive drum side as the imaging optical system for imaging the beam deflected for scanning by the deflection surface in a main scanning direction on the surface to be scanned.
In addition, in a sub-scanning section, principal rays of a plurality of beams from the multibeam light source are set parallel to an optical axis between the scanning lens 11a and the scanning lens 11b. Further, the beams from the multibeam light source are condensed at a focus position on the photosensitive drum side of the scanning lens 11b. In other words, Ls=fs2 is satisfied, where Ls represents a distance in the optical axis direction between a position of the scanning lens 11b and a position where the principal rays of the plurality of beams intersect each other (principal ray intersection position), and fs2 represents a focal length of the scanning lens 11b. In this way, Japanese Patent Application Laid-Open No. 2008-268721 discloses a structure in which a separation amount between the scanning lens 11b and the principal ray intersection position is set large.
However, if a plastic lens is used for the imaging optical system in the optical scanning apparatus described in Japanese Patent Application Laid-Open No. 2008-268721, a new problem occurs as described below. Specifically, when the plurality of beams from the multibeam light source pass through the plastic lens, if the plurality of beams are separated from each other in the sub-scanning direction, spot diameters of the individual beams are different because of an influence of birefringence generated in molding of the plastic lens.